An inkjet recording method is a printing method for performing printing by flying small droplets of an ink composition and making them adhere to a recording medium such as paper. This inkjet recording method has a characteristic feature that an image with high dissolution and high definition can be printed at a high speed. In general, an ink composition to be used in the inkjet recording method contains an aqueous solvent as a major component and further contains a colorant component and a wetting agent such as glycerin for the purpose of preventing clogging from occurring.
Also, in the case where printing is performed on a recording medium into which an aqueous ink composition hardly penetrates, for example, papers and cloths, or plates or films manufactured from a raw material of a metal or a plastic in which an aqueous ink composition does not penetrate, for example, phenol resins, melamine resins, vinyl chloride resins, acrylic resins and polycarbonate resins, the ink composition is required to contain a component capable of stably fixing a colorant to the recording medium. In particular, in the case where printing is performed on a printed wiring board, etc., the ink composition is required to have quick-drying properties or chemical resistance.
In order to meet these requirements, an ink composition containing a component which is polymerized upon irradiation with ultraviolet rays has hitherto been proposed (see, for example, Patent Document 1). An ultraviolet ray-curable ink composition containing a colorant, an ultraviolet ray absorber, a photopolymerization initiator and so on has also been proposed (see, for example, Patent Document 2). According to these ink compositions and inkjet recording methods, it is considered that bleeding of the ink composition into the recording medium can be prevented, thereby enhancing the image quality.
In the foregoing inkjet recording method using an ink composition containing a component which is polymerized upon irradiation with ultraviolet rays, after making the ink composition adhere to the recording medium, ultraviolet rays are irradiated. Then, the photopolymerization initiator in the ink composition produces a radical, etc., whereby an oligomer or a monomer starts polymerization and is cured. Therefore, the colorant in the ink composition is fixed onto the recording medium. It is thought that according to this fixing, printing with high film strength, solvent resistance and color density and less bleeding and unevenness can be realized.
As to the related-art ultraviolet ray-curable ink compositions, in general, those having high curing properties were low in storage stability, and a viscosity thereof increased with a lapse of time. When stored at high temperatures, they caused gelation so that not only they could not be applied to inkjet recording, but they could not be used by other recording methods. Conversely, when the storage stability was increased, the curing properties were lowered, and strong irradiation with ultraviolet rays was necessary. As a result, the device became large in size, or the consumed electric power increased. Therefore, such was not favorable.
It is thought that this is caused due to the matter that not only the photopolymerization initiator in the ink composition generates a radical, etc. due to irradiation with ultraviolet rays, but it generates a radical, etc. by thermal energy.
Patent Document 1: JP-A-3-216379
Patent Document 2: U.S. Pat. No. 5,623,001